In the fabrication of semiconductor integrated circuit (IC) devices, hundreds of fabrication steps must be performed on a semi-conducting substrate in order to complete the fabrication of the devices. The hundreds of processing steps may include cleaning, deposition, etching, buffer coating and various other necessary steps. In these fabrication steps, a variety of process chemicals, including liquids and gases must be used in different processing machines and be transported from their storage tanks to the machines. A large number of these process liquids are of high viscosity and short shelf life and therefore their transportation between a reservoir and a process machine must be carefully controlled. Deterioration or premature reaction of these process liquids can result in poor quality products and unnecessary machine down time which in turn lead to a decrease in process yield.
One of the process liquids that requires delicate handling is a photoresist stripper liquid. A photoresist stripper liquid, in order to be effective in stripping a photoresist layer from an IC device, must be heated to higher than room temperature, i.e. to as high as 80.degree. C. or preferably to as high as 115.degree. C. A typical photoresist liquid is highly flammable and therefore must be carefully controlled during the heating process. For instance, a widely used commercial photoresist stripper of ACT.RTM.-690 which contains various low boiling point and high boiling point components is shown in FIG. 1. The low boiling point components, i.e. those having a boiling temperature of up to 100.degree. C., includes dimethyl sulfide, IPA and acetone. The high boiling point components include dimethyl sulfoxide, N-methyl pyrolidone and methyl ethyl alcohol.
A conventional process tank for holding and heating a photoresist liquid is shown in FIG. 2. The process tank 10 is constructed by an inner tank 20, an outer tank 30 and an overflow tank 22 for the inner tank 20. The overflow tank 22 may be advantageously formed integral with the inner tank 20 to prevent overfill of the inner tank 20. The inner tank 20 is filled with a photoresist stripper liquid 24 which is heated by resistive heating elements 26 positioned at near the bottom 28 of the inner tank 20. The resistive heating elements 26 are normally constructed of a metallic heating element embedded in an insulating ceramic coating (not shown). During a regular heating mode, the surface of the resistive heating elements reaches a temperature between about 800.degree. C. and about 1000.degree. C. When the resistive heating elements 26 are properly controlled by a process controller (not shown), the photoresist stripper liquid 24 can be suitably heated to a temperature of about 125.degree. C., or to a temperature of at least 80.degree. C. For more efficient photoresist removal, the higher temperature of 115.degree. C. is more preferred. The inner tank 20 is further equipped with a fluid level indicator 32 which senses the level of the photoresist liquid being stored in the inner tank 20. The overflow tank 22 is further equipped with a set of level sensors 34 for sensing a high level, a normal level and a low level of the photoresist liquid in the overflow tank 22.
Surrounding the inner tank 20 and the overflow tank 22 for holding the photoresist stripper liquid, is an outer tank 30 which is equipped with a water inlet 36 such that the tank is filled with water 38 to a level that at least covers the bottom 28 of the inner tank 20. The use of deionized water is more preferred. The level of water 38 being held in the outer tank 30 is controlled by a fluid level indicator 40 to ensure that the bottom 28 of the inner tank 20 is always immersed in the water 38. The upper level of water 38 is controlled by a drain pipe 42 which drains away overfilled water 38 in the outer tank 30. The outer tank 30 is further equipped with a manual drain valve 44 which is used to completely drain the outer tank 30 when maintenance or cleaning of the tank is required. The inner tank 20 is supported by a plurality of supports 46 positioned on the bottom panel 48 of the outer tank 30. The outer tank is further equipped with an ultrasonic vibration device (not shown) such that ultrasonic vibration can be transmitted to the inner tank 20, i.e. thus to the photoresist stripper liquid 24, to facilitate mixing and to achieve a more uniform temperature in the stripper liquid. The ultrasonic vibration from the outer tank 30 is transmitted to the inner tank 20 by water 38 that contacts at least the bottom 28 of the inner tank 20.
The sidewalls 18 and the bottom wall 28 of inner tank 20 should be constructed in a high temperature resistant material for holding the heated, highly flammable photoresist liquid 24. A suitable material that is chemically inert, dimensionally stable for holding such photoresist stripper liquid heated to a temperature of 115.degree. C. is quartz. The use of a ceramic material such as quartz for forming the inner tank 20 presents a serious problem of breakage due to the fragile nature of quartz upon impact or vibration. When the quartz inner tank 20 breaks or fractures, the photoresist stripper liquid 24 leaks out of the inner tank 20 into the outer tank 30 and mixes with water 38 to be drained out through drain pipe 42 to a fluid level 16 that is below the resistive heating elements 26 and thus leaving the heating elements 26 exposed. When the resistive heating elements 26 are no longer submerged in liquid 24, the high surface temperature of the heating elements, i.e. as high as 1000.degree. C., immediately causes a fire potential by igniting residual photoresist stripper liquid on the heating elements 26, or by igniting the residual vapor left in the inner tank 20 even after the photoresist liquid 24 is substantially drained away. Any such fire would cause a disastrous effect since a number of photoresist stripper tanks may be positioned close to each other, and furthermore, the inner tank 20 normally has a capacity of about 50 liters of the highly flammable liquid. The potential for a severe fire that is difficult to control is therefore very high upon a breakage of the inner tank and must be protected.
It is therefore an object of the present invention to provide an apparatus for preventing fire in a liquid heating tank that does not have the drawbacks or shortcomings of the conventional apparatus.
It is another object of the present invention to provide an apparatus for preventing fire in a liquid heating tank which holds a highly flammable photoresist stripper liquid.
It is a further object of the present invention to provide an apparatus for preventing fire in a liquid heating tank that utilizes resistive heating elements which would be exposed upon a breakage of the tank and cause a fire.
It is another further object of the present invention to provide an apparatus for preventing fire in a liquid heating tank that utilizes resistive heating elements by providing a smaller buffer tank positioned below the liquid heating tank which is equipped with an air solenoid valve for shutting off a drain pipe and thus stopping the photoresist stripper liquid/water mixture from draining from an outer tank.
It is still another object of the present invention to provide an apparatus for preventing fire in a liquid heating tank that utilizes resistive heating elements for heating flammable liquid which is capable of keeping resistive heating elements submerged in the flammable liquid and preventing ignition of the flammable liquid.
It is yet another object of the present invention to provide an apparatus for preventing fire in a liquid heating tank that utilizes resistive heating elements by using a buffer tank positioned under the liquid heating tank for sensing a temperature rise in the buffer tank and shutting off a drain valve such that the resistive heating elements remain submerged in the liquid.
It is still another further object of the present invention to provide a method for preventing fire in a tank for heating a flammable liquid by preventing the exposure of bare heating elements to the flammable liquid or vapor.